Coated paper is used for a large range of products including packaging, board art paper, brochures, magazines, catalogues and leaflets. Such coated paper is required to give a range of properties, including brightness, opacity and sheet gloss, as well as printing performance.
Paper coating compositions are generally prepared by forming a fluid aqueous suspension of particulate pigment material together with a binder and other optional ingredients. The coating may conveniently be applied by means of a coating machine including a short dwell time coating head, which is a device in which a captive pond of coating composition under a slightly elevated pressure is held in contact with a moving paper web for a time in the range of from 0.0004 second to 0.01 second, before excess coating composition is removed by means of a metering blade. However, other types of coating apparatus may also be used for preparing coated paper. The coated paper is required to meet certain standards of surface gloss and smoothness. For example, the paper is generally required to have a gloss value of at least about 32, and up to about 70, in some cases up to about 90 TAPPI units, and a Parker Print Surf value in the range of from about 0.5 μm to about 1.6 μm.
It is known to use particulate calcium carbonate, either alone or in conjunction with other pigment material such as, for example, processed particulate kaolin clay, as a particulate pigment in paper coating compositions.
Particulate calcium carbonate can be obtained from natural sources or can be manufactured synthetically. Manufactured calcium carbonate is generally obtained by precipitation from aqueous solution. Precipitated calcium carbonate (PCC) is obtained in three different principal crystal forms: the vaterite form, which is thermodynamically unstable, the calcite form which is the most stable and is also the most abundant natural crystalline form, and the aragonite form which is metastable under normal ambient conditions of temperature and pressure, but converts to calcite at elevated temperatures.
The aragonite form typically crystallises as long, thin needles (acicular shape) having a typical length:diameter ratio of about 10:1, but the calcite form exists in several different shapes, of which the most commonly found are: the rhombohedral shape in which the crystals may be either aggregated or unaggregated; and the scalenodedral shape, in which the crystals are like double, two-pointed pyramids having a typical length:diameter ratio of about 4:1, and which are generally aggregated. All these forms of calcium carbonate can be prepared by carbonation of an aqueous lime-containing medium by suitable variation of the process conditions.
Calcium carbonate can be ground to obtain particulate ground calcium carbonate (GCC), by methods which are well known in the art.
Blends of PCC and GCC, and of different GCCs, for use in paper coating are known in the art.
For instance, U.S. Pat. No. 5,879,442, the disclosure of which is incorporated herein by reference, describes an aqueous slurry of calcium carbonate for use as a paper coating pigment, the slurry containing from 70 to 85% by weight of a combination of PCC particles and GCC particles in a weight proportion in the range 20:80 to 80:20, advantageously from 51:49 to 70:30. The calcium carbonate particles according to this patent have a median diameter in the range 0.2 to 2.0 μm, preferably from 0.3 to 1.5 μm. This patent, however, does not disclose the particle size distribution for the calcium carbonate particles.
Similarly, PCT Patent Application No. WO-A-01/53893, the disclosure of which is incorporated herein by reference, describes a printing paper having an image-receptive coating comprising a binder and a non-platy pigment. Examples are given (e.g. Examples A to D) of a non-platy pigment which comprises (out of 100 parts) 15 parts GCC, 75 to 79 parts PCC and 6 to 10 parts of a hollow sphere plastic pigment. This patent application discloses that, for glossy coated printing sheets, preferably about 90 to 95% of the calcium carbonate particles have an equivalent spherical diameter (esd) less than 2 μm and that, for matte coated printing sheets, preferably about 65% of the particles have an esd of less than 2 μm. The patent application also describes the calcium carbonate pigment exhibiting a relatively narrow particle size distribution, but it does not disclose what this is relative to. Further, details of the particle size distribution are not given.
The present invention comprises the discovery that improved properties can be obtained when paper is coated with a paper coating composition which includes a pigment comprising a blend of a selected particulate PCC and a selected particulate GCC or a blend of selected particulate GCCs. By using a blend of selected particulate PCC and GCC, a synergistic improvement to the opacity, brightness and print gloss of the paper has been discovered, when compared to paper in which the pigment in the coating is either one of the individual components of the blend.